A method of forging a rod-shaped work includes a known method of forming an expanded portion in at least an axial-directional portion of a metal rod- shaped work by an upset forging process. The "rod-shaped work" is defined as a rod-shaped member which is molded by an upset forging process.
The foregoing conventional method has steps of setting a rod-shaped work into a forging die, applying a load to the rod-shaped work in an axial direction thereof and charging a material of the rod-shaped work into an expanding cavity of the forging die so that an upset forging process is performed, by which an expanded portion is formed in an axial-directional portion of the rod-shaped work.
In Japanese Utility-Model Laid-Open No. 58-147643 and Japanese Patent Laid-Open No. 7-1068, methods have been disclosed with which an upset forging process is performed to manufacture a plastic-region clamping bolt which is also called a tension bolt.
When an expanded portion is formed by upset-forging a rod-shaped work, a deep underfill portion is sometimes generated in the expanded portion. The deep underfill portion is generated in the expanded portion where the work-forming material has not sufficiently filled the expanded cavity of the forging die. Since the molding load required to charge the work-forming material is increased in proportion to the upsetting length in the axial direction of the rod-shaped work, the frequency of generation of the deep underfill portions, which cannot be removed by a post-process, such as a drawing or ironing process is increased.
The molding load required to charge the work-forming material for the work is increased in proportion to an upsetting ratio (upsetting diameter after the upsetting/diameter of the material before the upsetting). Therefore, the frequency of generation of the deep underfill portions, which cannot be removed by a post-process, is increased.
The reason why the deep underfill portion, which cannot be removed by a post-process, is generated will now be described in reference to a conventional example for upset-forging a usual rod-shaped work. As schematically shown in FIGS. 9a and 9b, a rod-shaped work 1 is buckled into a wedge shape as the rod- shaped work 1 is compressed. Thereafter, further wedge-shape buckling proceeds. It is noted that FIGS. 9a and 9b are based on the drawing of "Press Working Guide", p.p. 543, edited by Japan Plastic Work Society and published by Maruzen, Co., Ltd. on Oct. 25, 1975.
Increasing the molding load is an effective way to prevent generation of the deep underfill portion. If the molding load is too high, however, the forging die can easily be broken. Therefore, in the actual operation, generation of the deep underfill portion is permitted to some extent. The generated deep underfill, if any, is judged as a defect that must be removed in a post-process. Thus, rather than increasing the molding load to prevent generation of the deep underfill, the molding load is kept low to ensure a long service life of the forging die, even though this results in deep underfills that form defective parts.